Method for preventing edge defects in compression stretched acrylic sheets

ABSTRACT

A METHOD WHICH COMPRISES PROVIDING AN ACRYLIC BLANK WITH A CIRCUMFERENTIAL GROOVE HAVING A DEPTH OF AT LEAST 5% OF THE INITIAL THICKNESS OF THE BLANK AND THEREAFTER COMPRESSING THE BLANK TO CAUSE THE ACRYLIC MATERIAL TO FLOW OUTWARDLY FROM THE CENTER OF THE BLANK TO PRODUCE A COMPRESSION-STRETCHED SHEET HAVING LESS THICKNESS THAN THE BLANK. THIS METHOD REDUCES EDGE DEFECTS IN COMPRESSION STRETCHED ACRYLIC SHEETS.

Feb. 9, 1971 R, L AYRES 3,562,383

METHOD FOR PREVENTING EDGE DEFEGTS IN COMPRESSION STRETCHED ACRYLICSHEETS Filed Sept. 9, 1968 /g/WENTOR.

rrafA/EKS United States Patent O1 Ecc 3,562,383 Patented Feb. 9, 1971U.S. Cl. 264-296 8 Claims ABSTRACT F THE DISCLOSURE A method whichcomprises providing an acrylic blank with a circumferential groovehaving a depth of at least of the initial thickness of the blank andthereafter compressing the blank to cause the acrylic material to flowoutwardly from the center of the blank to produce acompression-stretched sheet having less thickness than the blank. Thismethod reduces edge defects in compression stretched acrylic sheets.

BACKGROUND OF THE INVENTION This invention relates to stretching ofacrylic sheets and, more particularly, to the prevention of edge defectsin stretched acrylic sheets.

Because of its strength, formability, and light transmissioncharacteristics, acrylic material has a variety of uses and particularlyhas been used to form windows and canopies for aircraft. Manufacture ofthe acrylic part often involves a stretching step in which the acrylicmaterial is heated and stretched to enhance its physical characteristicsand/or to provide the acrylic part with a required curvature.Heretofore, stretching of acrylic blanks has been accomplished by eitherpulling the edges of the acrylic blanks in opposing directions such asis described in U.S. Pat. Re. 24,978 of Bottoms et al. or of compressionstretching acrylic blanks as described in U.S. patent application Ser.No. 691,294 of Terry D. Fortin, tiled on Dec. 18, 1967. Regardless ofthe way in which the acrylic blanks are stretched, stresses are set upin the edges 0f the acrylic blanks which cause cracking and subsequentloss of material around the edges of the acrylic blanks. Additionally,such edge cracks can propagate throughout the stretched sheet, therebyresulting in a near total loss of the acrylic part.

To reduce losses from acrylic blanks stretched by the tension methoddescribed in the above-mentioned Bottoms patent, the edges of theacrylic blanks have been cooled relative to the remainder of the acrylicblank as the acrylic blank is stretched. This method has reduced thematerial lost when stretching by this method. However, other stressesare produced in the stretched acrylic sheet due to the temperaturedifferential between the edge and interior sections of the stretchedacrylic part.

To the best of the knowledge of the instant inventor, no methods havebeen previously used to prevent edge losses when stretching acrylicblanks by the compression method described in the aforementioned Fortinapplication.

SUMMARY OF THE INVENTION The method of this invention comprisesproviding the circumferential end surfaces of acrylic blanks with asubstantially continuous groove therein. The depth of the groove is atleast about 5% and, preferably, is greater than about 10% of the initialthickness of the acrylic blank. The groove may have a variety ofconfigurations such as, for example, V- and U-shaped configurations andthe centerline of the groove may be offset from the centerline of thecircumferential end surfaces. However, it is preferable to position thegroove so that its center-line corresponds with the center-line of thecircumferential end surfaces.

The grooving of acrylic blanks as described is relatively easy toaccomplish and the machining producing the groove apparently sets up nonoticeable stresses in the edges of the acrylic blanks. In spite of itssimplicity and ease of accomplishment, the method of this invention hasbeen extremely successful in substantially eliminating losses from theedges of compression stretched acrylic sheets.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. l is a perspective view of anunstretched acrylic blank showing one form of circumferential end grooveof this invention formed therein;

FIG. 2 is a partial perspective view of a compressionstretched acrylicsheet which was stretched from an acrylic blank having nocircumferential end groove therein (i.e., prior art);

FIG. 3 is a partial perspective view of a compressionstretched acrylicsheet stretched from the acrylic blank of FIG. l;

FIG. 4 is a partial sectional view of the unstretched acrylic blank ofFIG. 1 taken along the lines 4-4 of FIG. l;

FIG. 4a is a sectional view of the stretched acrylic blank of FIG. 3taken along the lines 4a-4a of FIG. 3;

FIG. 5 is a sectional view of an acrylic blank provided with anotherform of circumferential groove of this invention but different in shapefrom the groove of FIG. 4;

FIG. 5a is a sectional view of that portion of the acrylic blank shownin FIG. 5 after being compression-stretched;

FIGS. 6, 7 and 8 are sectional views of unstretched acrylic blankshaving variously shaped and positioned I circumferential end grooves ofthis invention; and

FIG. 9 is a partial elevational view of a stretched acrylic sheetshowing edge roll produced from compression stretching.

DESCRIPTION OF THE PREFERRED EMBODIMENT The method of this inventioncomprises providing the circumferential end surfaces of acrylic blankswith a groove which is substantially continuous therewith. The depth ofthe groove is at least about 5% and, preferably is at least about 10% ofthe initial thickness of the acrylic blank. Various congurations, e.g.,V- and U-shaped and rectangular configurations, may be used. Preferably,the center line of the groove lies along the center line of the acrylicblank taken in the thickness direction.

Acrylic blanks having the aforedescribed groove may be heated andcompression stretched as described in the U.S. application Ser. No.691,294 of Terry D. Fortin, filed Dec. 18, 1967 (hereafter referred toas said Fortin application). When acrylic blanks are grooved andstretched in this manner, the resultant stretched acrylic sheets exhibitsubstantially no edge defects. Therefore, the loss of acrylic materialis substantially eliminated. In acrylic blanks which are not providedwith the groove of this invention, the edges of the acrylic blanks,after being stretched by the method described in said Fortinapplication, often are highly stressed resulting in weekened or crackededges (FIG. 2).

Acrylic blanks stretched according to said Fortin application are heatedto their softening temperature range and, thereafter, compressed tocause the acrylic material to ow or squeeze radially outwardly to form astretched acrylic sheet of reduced thickness. It is believed that theundesirable edge defects result from the partially molten edges of theacrylic blanks rolling non-uniformly outwardly from the central portionof the acrylic blanks. This rolling of the outer edges apparently takesplace at a rate faster than the radially outward movement of theremainder of the acrylic blank. This differential rate of movement isbelieved to be the cause of the stresses and cracks which are introducedinto the edges of the stretched acrylic sheet. The success of the grooveof this invention in preventing fracturing of the edges of compressionstretched acrylic sheet is believed due to the restraining of this edgeroll due to a change in edge forces which is, in some manner, derivedfrom the aforedescribed circumferential grooving. That is, it ishypothesized that the initial compression forces exerted on the groveexceed the non-uniform outward forces which tend to create edge roll,thereby restraining outward flow of the edge material so that itsoutward flow is substantially equal to the outward flow of the remainderof the acrylic blank as the blank is stretched.

The method of this invention will now be more particularly describedwith reference to the figures. In FIG. l, the numeral 10 designates anas-cast acrylic blank which has a circumferential end surface 11. Theterm acrylic blank will be employed herein, and in the claims, to denoteunstretched acrylic material. The end surface 11 of the blank 10 isprovided with a continuous groove 12. Some improvement in the edges ofcompression-stretched acrylic sheet may be produced regardless of theshape and size of the groove 12 in the acrylic blank 10. However, it hasbeen found that if the depth of the groove is less than about of theinitial thickness of the acrylic blank to be stretched some cracking andsplitting of the edges always occurs. Above about 5% but below about ofthe original thickness, some edge roll is still present and the latteris sometimes accompanied by limited cracking of the acrylic material.However, there is significant improvement as compared with stretchedacrylic in which no circumferential grooves were formed. By comparison,acrylic sheets stretched from blanks having circumferential end groovesof depth greater than about 10% of the blank thickness, substantiallyalways exhibit no edge defects.

As used herein, the term edge defects refers to a random, unevenstressing, cracking and/or breaking of the edges of stretched sheets asshown, by way of example, in FIG. 2. Such edge defects can result insubstantial loss of acrylic material. While the shallow circumferentialgrooves which may be present around the edges of acrylic material afterit has been stretched by the herein-described method, such as shown inFIG. 3, and constitutes a loss of material, such loss is only a minorfraction of that due to uneven stresses and other edge cracking. Thisloss is minimal because the shallow grooves extend a uniform distanceinto the material and are not accompanied by cracking, stressing, etc.,which would necessitate the removal of good material in order to removestressed or cracked as in the case with stretched sheets exhibiting edgedefects.

A section 13 of an acrylic sheet originally having a shape and edgegroove 12 such as is shown in FIG. 1, is shown in FIG. 3 after beingcompression-stretched. In addition to having a reduced thickness ascompared with the blank 10 of FIG. l, the stretched sheet 13 has a muchsmaller edge groove 14. The smaller groove 14 is produced by the fillingin of the original groove 12 as the acrylic blank 10 iscompression-stretched. The latter is further shown by comparing thegrooves 12 of FIGS. 4 and 5 with the grooves 14 of FIGS. 4a and 5a,respectively. As will be seen from this comparison, the stretched sheets13 of FIGS. 4a and 5a have substantially smaller grooves 14 than theacrylic blanks 10 of FIGS. 4 and 5.

It has also been found that the most advantageous results are obtainedwhen the grooves in the circumferential end surfaces of the blanks aresymmetrical about the plane passing through the center-line of suchsurfaces as shown in FIGS. 4, 5 and 6. That is, optimum results areproduced when the maximum depth of the grooves is located at thecenter-line of the circumferential surfaces and the center-line of thegroove corresponds to the center-line of the circumferential surfaces.With this optimum positioning of the grooves in the blanks, the materialwhich has to be removed from the edge of the stretched sheet to removeany shallow grooves remaining after compression is minimized, ascompared with an asymmetrical groove as shown in FIGS. 7 and 8.

The shape of the groove does not appear to be critical. Grooves ofvarious cross-sectional shapes, c g., V-shaped, U-shaped, circular andrectangular (FIGS. 4, 5 and 6) have been successfully employed.Additionally, the mouth of the groove may be as wide as, or narrowerthan, the width of the circumferential end surfaces, as shown in FIGS. 5and 8. The groove is substantially coextensive with the length of thecircumferential end surface (FIG. l) since edge defects may beintroduced into the stretched sheet at any point along the end surfaceif there is no grooving at that point.

In actual stretching operations, it is preferable to provide a testblank with a groove having a particular depth (greater than 10% of thethickness of the blank) and, thereafter, to compression stretch the testblank to the desired stretched size. By examining the depth of thegroove in the stretched part, it can be determined whether to useshallower or deeper grooves in the production blanks. For example, if arelatively deep groove remains in the stretched sheet, a shallowergroove than initially present in the test blank can be formed or cut inthe production blanks so that edge losses from the stretched part willbe further minimized.

Cracking and stressing of the edges is generally preceded by edge rollas shown in FIG. 9. Thus, if edge roll is present along the edges of astretched acrylic sheet, a border-line condition is indicated and thegroove placed in the acrylic blank should be deepened.

Optimally, the acrylic blank is provided with a groove having a depth ofat least about 5% and preferably greater than 10% of the original blankthickness and, further, which will be substantially eliminated in thestretched acrylic sheet. That is, optimally the depth of the acrylicblank groove is selected so that the flow of material during compressionstretching substantially fills y the groove. Such groove selection, ofcourse, minimizes or eliminates any losses of material from thestretched sheet.

Following the choice of a satisfactory groove depth, the productionacrylic blanks are provided with edge groove by any method, e.g.,sawing, which will not critically stress the blank. The blanks are thenready to be compression stretched. Compression stretching of the groovedblanks may be performed by any desired technique. However, it ispresently preferable to employ the compression stretching methoddescribed in said Fortin application, the description of which isincorporated herein by reference to said Fortin application.

Examples of the article and process of this invention follow:

EXAMPLE 1 An as-cast Plexiglas 55 blank (manufactured by Rohm & Haas)having initial dimensions of 6 in. X 6 in. x 0.751 in. thick wasprovided with a circumferential groove having a configurationsubstantially as shown in FIG. 8. The maximum depth of the groove was0.125 inch, or about 16% of the initial acrylic blank thickness. Thisblank was coated with Mold-Wiz F-57 lubricant (Teflon colloidaldispersion manufactured by Axel Plastics Research Lab., Inc.) and placedbetween a pair of polished glass plates, and isothermally heated to atemperature of 295 F.

The blank was thereafter compressed at a thickness reduction rate ofabout 0.10 inch per minute to produce a stretched acrylic sheetmeasuring 9.5 in. X 9.5 in. x 0.270 in. thick. Thereafter, thetemperature of the acrylic sheet was reduced at a rate of 13 F. perminute to a temperature below about F.

No cracks developed in the stretched acrylic sheet and no edge roll wasobserved. Additionally, the size of the groove in the blank `wassubstantially reduced.

EXAMPLE 2 A blank of the same material and of the same size as used inExample 1 was provided with a V-shaped circumferential groove similar tothat shown in FIG. 4 and having a maximum depth of 0.1 inch or about 13%of the acrylic blank thickness. This blank was stretched undersubstantially the same conditions as described in Example 1 to astretched acrylic sheet having dimensions of 9 in. x 9 in. x 0.350 in.thick.

No cracks or edge roll were observed in the stretched acrylic sheet andthe depth of the original groove in the acrylic blank was substantiallyeliminated in the stretched sheet.

EXAMPLE 3 An acrylic blank formed from the same material and having thesame dimensions as that described in Example 2 was provided with aV-shaped groove similar to that used in `Example 2 except that themaximum depth of the groove was 0.046 inch or about 6% of the acrylicblank thickness. This blank was stretched under the same conditions asemployed in Example 2 to a stretched acrylic sheet having dimensions of9.5 in. x 9.5 in. x 0.300 in. thick.

No cracks were observed in the edges of the stretched acrylic sheet.However, some edge roll was noticed along the edges of the sheet and,from experience, this indicated a. borderline condition.

EXAMPLE 4 An 'as-cast acrylic blank of Plexiglas 55 having dimensions of12.5 in. x 12.5 in. x 0.732 in. was provided with a circumferentialgroove of square cross-section measuring 0.187 inch in depth (about 25%of the original blank thickness) and positioned substantially as shownin FIG. 5. This acrylic blank was compression stretched employing themethod of Example 1 to a stretched acrylic sheet having dimensions of21.6 in. x 21.6 in. x 0.245 in.

No cracks or edge roll were observed along the edges of the stretchedacrylic sheet.

EXAMPLE 5 An acrylic blank of Plexiglas 55 having dimensions of 12.5 in.x 12.5 in. x 0.740 in. was provided with a continuous circumferentialgroove similar to that shown in FIG. 7 and having a maximum depth ofabout 0.375 inch (about 50% of the initial blank thickness). This blankwas compression stretched by the method of Example 1 to a stretchedacrylic sheet having dimensions of 22.2 in. x 22.2 in. x 0.233 in.

No cracks or edge roll were observed along the edges of the stretchedacrylic sheet.

EXAMPLE 6 An 'as-cast acrylic blank of Plexiglas 55 having dimensions of12.5 in. x 12.5 in. x 0.735 in. was provided with a continuouscircumferential V-shaped groove as shown in FIG. 4 and having a maximumdepth of about 0.312 inch (about 40% of the initial blank thickness).This acrylic blank was stretched by the method of Example 1 to astretched acrylic sheet having dimensions of 21 in. x 22 in. x 0.235 in.

No cracks or edge roll were observed along the edges of the stretchedacrylic sheet.

As will be apparent from the foregoing, modifications in the presentinvention may be made by those skilled in the art without departing fromthe spirit of the invention. Therefore, this invention is to be limitedonly by the scope of the claims which follow.

I claim:

1. A method for compression stretching an acrylic blank having a top anda bottom surface and circumferential end surfaces extendingtherebetween, comprising the steps of:

providing said circumferential edge surfaces of said acrylic blank witha groove having a depth of at least about 5% of the initial thickness ofsaid blank;

and compressing said top and said bottom surfaces of said blank to causesaid blank to stretch radially outwardly from the center of said blankto produce a compression-stretched sheet having less thickness than saidblank.

2. The method of claim 1 wherein the depth of said groove is greaterthan about 10% of said initial thickness of said blank.

3. The method of claim 1 wherein said groove is substantiallycoextensive with said circumferential end surfaces.

4. The method of claim 1 wherein said groove is symmetrical about aplane through the center-line of said circumferential end surfaces.

5. The method of claim 1 wherein said acrylic blank is selected from thegroup consisting of methyl methacrylate and modified methylmethacrylate.

6. The method of claim 1 wherein, in addition, said depth of said groovein said blank is selected so that said groove is substantially lled insaid compression-stretched sheet.

7. A method of compression stretching an acrylic blank having a top anda bottom surface and a circumferential end surface therebetweencomprising the steps of providing said circumferential end surfaces witha groove substantially coextensive therewith, said groove having a depthgreater than about 5% of the initial thickness of said blank andpositioned symmetrically about a plane passing through the centerline ofsaid groove;

heating said acrylic blank to a temperature within the softeningtemperature range of said acrylic material;

compressing said heated acrylic material between a pair of polishedelements, having a thin film of lubricant coated thereon, to reduce thethickness of said acrylic material at a rate which producessubstantially uniform ow, without cracking, of said acrylic material;

Cooling the resulting stretched acrylic material at a predetermined rateto prevent cracking of said stretched acrylic material to a temperaturebelow the minimum softening temperature of said acrylic rnaterial; and

releasing said pressure after said stretched acrylic material hasattained a temperature :below said minimum softening temperature,whereby stretched dacrylic material is produced which has improvedoptical and physical properties.

8. The method of claim 7 wherein said depth of said groove is at leastabout 10% of said initial thickness of said blank.

References Cited UNITED STATES PATENTS 2,166,215 7/1939 Lloyd 264-12,319,014 5/1943 Smith 264-1 2,332,674 10/ 1943 Smith 264-1 FOREIGNPATENTS 882,003 11/1961 Great Britain 161-149 JULIUS FROME, PrimaryExaminer H. MINTZ, Assistant Examiner U.S. Cl. X.R.

